As is known in the art, commercial and military telecommunications systems over the past few years have shown a trend toward using digital modulation techniques. These digital systems require the capacity to handle a high density of carrier frequencies in order to remain cost effective. The additional trend toward space based systems imposes high efficiency and weight minimization constraints. Power amplifiers represent an important design challenge as they must conform to the above specifications. Unfortunately, high efficiency in power amplifiers has been difficult to attain for a large number of carrier frequencies. In fact, this is a direct trade-off in conventional amplifier design.
As is also known in the art, in telecommunications systems, satellite systems and other systems, it is desirable for radio frequency (RF) power amplifiers (PAs) to linearly amplify RF signals in a highly efficient manner. Efficiency is generally a function of RF output power level, and thus also a function of RF input level. For amplifier configurations with good efficiency performance, the efficiency varies approximately proportional to the square root of the RF output power level. High efficiency is typically not attained until an amplifier approaches its maximum output power. This, however, is not consistent with linear operation. Thus, a tradeoff must typically be made between achieving maximum efficiency and high linearity in RF power amplifier circuits.
Techniques suggested include reducing bias conditions for the RF transistors in the amplifier, or shutting off of some portion of the transistor in the amplifier. These approaches provided only minimal improvement in efficiency in the back-off power output region, i.e., the region backed off from the saturation region. One approach to reduce bias is to provide a switched loadline. One switched loadline technique uses a PIN diode switch in the output matching network. However, the PIN diode switch requires DC current in one of the on-off states and also adds RF loss that causes degradation of some of the efficiency gains.